The phrase “not too thick” is correct and usable in written English.
You can use it when you need to describe something that isn’t very thick. For example, “I like my pancakes not too thick.”.
These are not too thick on the ground, though.
The batter should be fairly liquid and not too thick.
It's not too thick and not too thin.
You're going for a relatively even layer that's not too thick.
Ideally, it should be runny but not too thick as to be cloying.
"You have to put the marshmallow on a nice, long stick -- not too thick, though," she cautioned.
Not too thin, not too thick and fried at a temperature that causes it to puff out and crisp simultaneously.
The dressing on the Caesar salad was tasty and not too thick, with a thick dusting of grated pecorino.
The bread We use a simple white sandwich loaf from the St John bakery, sliced not too thick.
In recent years, physicists have found that people invariably prefer a certain mathematical density of fractals — not too thick, not too sparse.
